1. Field of the Invention
This invention is directed to a thermistor for temperature measurement, control and/or temperature compensation and a method of making the same. More particularly, the invention is directed to a thermistor having multiple layers of electrode metal.
2. Background of the Invention
Thermistors (thermally sensitive resistors), are ceramic semiconductors which exhibit large changes in electrical resistance with corresponding changes in temperature. Because of their sensitivity, accuracy, and stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement, compensation, and control. Thermistors are used extensively for commercial consumer electronic products, automotive, industrial, and medical electronic applications, food handling and processing, communication and instrumentation, computers, military and aerospace, and research and development. Some practical uses of thermistors include liquid level measurement, photography, thermometers, intravenous catheters, blood analysis, myocardial needle probes, automotive climate control, fuel level/temperature, temperature sensors for household appliances such as air conditioners, coffee makers, and data logger applications such as air, soil, liquid temperature probes. Their use in portable phones, earphones, quartz oscillators and transceivers continues to expand.
The most important characteristics of thermistors are an extremely high temperature coefficient of resistance and precise resistance versus temperature characteristics. The sensitivity to temperature change can result in a thermistor resistance change of 10 million to one over an operating temperature range. Prior art chip thermistors are of small size, square configuration, are available coated or uncoated, in leaded or unleaded embodiments, having operating temperature ranges of −80° C. to 300° C., and resistance ranges from 0.5 ohms to 40 megohms.
The electrical resistivity of a positive temperature coefficient (PTC) thermistor increases with increase in temperature. PTC thermistors switch from a low resistance to a high resistance state at a specific temperature. They are widely used as current limiters from −80° C. to 300° C., 0.5 ohms to 40 megohms. Conversely, the electrical resistivity of a negative temperature coefficient (NTC) thermistor decreases with increase in temperature. NTC thermistors are used to sense temperatures from −80° C. to 300° C. with nominal resistance at 25° C. from 0.5 ohms to 40 megohms. Therefore, they have a large temperature coefficient of resistance and a wide range of resistance values. They are also available in a wide range of sizes from 3 mm in diameter to 22 mm in diameter in adaptable shapes and sizes for a wide variety of mechanical environments. Typical applications for NTC thermistors include fan control, temperature sensing, circuit protection and temperature control. NTC thermistors are chosen when remote sensing is required, small size is desired, or where small temperature differences need to be measured.
NTC thermistors used for temperature measurement and compensation are usually made from various compositions including the oxides of manganese, nickel, cobalt, copper, iron, and other metals to form a ceramic semiconductor material. Thermistors may be formed into different shapes of bead, disc, rod, chip or flake configuration. The flake style thermistor is simply a much smaller size version of the chip thermistor. Wafer thermistors are produced by forming thin sheets of material including powders of the oxides of manganese, nickel, and other oxides combined in a binder. The material is sintered at elevated temperatures, coated with a conductive metal composition, and then diced to size. Leads are attached by soldering. The units are finally coated in an epoxy or other electrical insulation material for final protection and stabilization. A typical prior art thermistor element, shown in FIG. 1, represents a chip type thermistor composed of sintered powders of metal oxides (1) on which electrodes (2) and (3) are deposited.
Specifically, when the prior art thermistors with thick film electrodes made with Ag, PdAg or Au are attached to substrates (surface mount configurations, FIG. 1) or to leads (discrete component configurations, FIG. 2) with high temperature solders using processes operating between 200° C. and 380° C. at dwell times ranging from 5 seconds to 3 minutes, their electrical resistance shifts outside the allowable specified resistance tolerance (typically 2-5%). This results in a defective or deficient final product or sub-assembly into which the thermistor is assembled.
These resistance shifts of the prior art thermistors have now been found to be caused by a phenomenon called leaching, which occurs during the soldering process. Leaching occurs because the metal in the electrode has a higher affinity for the molten solder than its bond with glass frit or fritless binder of the electrode. As the thermistor electrode is being soldered, the metal is released from its bond with the glass frit or fritless binder of the electrode and is absorbed into the molten solder. As a result, the electrical resistance of the thermistor increases from its original value, prior to the soldering process. In other words, the metal element forming the external electrodes will be compromised due to the solder leaching.
The rate of leaching of the thermistor thick film electrode is dependent on the type of electrode material and the temperature and the duration of the soldering process to which the thermistor is exposed. Typically, exposing thermistors of the prior art to molten solder at temperatures above 200° C. for extended periods of time (greater than 5 seconds) is not recommended by thick film electrode manufacturers since degradation of the electrode increases more rapidly above this temperature and beyond this time. In addition to the shift in electrical resistance, leaching causes degradation of the solder-electrode and electrode-semiconductor bond. Weakened bonds may result in thermistors having greatly reduced stability and reliability.
Thick film Pt electrodes have been found to be resistant to leaching compared to other electrode materials. However, the high cost of thick film Pt electrodes renders the prior art thermistor not cost effective to manufacture. Also, it is more difficult to bond Au wire to thick film Pt electrodes using the thermo-sonic or equivalent wire ball bonding process.
In addition, prior art thermistors with thick film Ag or PdAg electrodes are not commonly used in hybrid microcircuit applications requiring 0.001″ OD gold wire (or equivalent) to be bonded using the thermo-sonic or equivalent wire ball bonding process because the wire bonds to these electrodes may not be reliable over the long term.
A thermistor element using two layers of thin film electrodes have been described in the prior art (U.S. Pat. No. 4,712,085). Other prior art (U.S. Pat. No. 6,008,717) describes a thermistor with a pair of electrodes in a shorter inner electrode and a longer inner electrode are mutually opposite each other and separated by a gap. However, this prior art does not solve the leaching problem described.